The present invention relates to a hands-free system which makes it possible to communicate without using a handset and which makes it possible to use a speaker and a microphone during a communication session; more particularly, the present invention relates to a voice switch used to control howling, which causes a problem in a hands-free system.
A hands-free system provides a function of communicating without using a handset and of allowing a communication session to be carried out by using a speaker and a microphone. The hands-free system is applied to telephones and to teleconference systems. In a hands-free system, it is generally required to provide a means for preventing howling from occurring when a gain of a local loop, formed by acoustic coupling between the speaker and the microphone, exceeds 1.
One of the embodiments of the above-mentioned means is a voice switch. A voice switch compares respective signal powers of a receive speaking circuit and a transmit speaking circuit, and determines that the signal having a greater power is associated with the current speaker, whereupon the voice switch applies a great attenuation to the speaking circuit determined not to be associated with the current speaker. This ensures that the gain of the above-mentioned local loop is below and that howling is prevented from occurring. Since the voice switch always applies a great attenuation to a signal on one of the speaking circuits, the communications provided by the voice switch are invariably one-way communications. Normally, the two parties engaged in a communication session take turns in speaking, so that the one-way communication does not render a conversation impossible. However, a voice switch is required to effect switching at a rate as rapid as possible in order to provide a conversation as smooth as one taking place when a handset is used.
An echo canceller is known as a means for resolving the problem arising from the above one-way communications. However, since it is required to allow a certain amount of convergence time in the echo canceller under variations of the acoustic coupling, it becomes necessary to use the echo canceller and the voice switch jointly in order to prevent howling from occurring due to reduction, during the convergence time, of an echo cancellation amount (rise in the gain of the local loop). That is, even when the echo canceller is introduced in the system, it is in great demand that a voice switch capable of rapidly switching the talking direction be provided in the system in order to achieve a conversation as smooth as one taking place when a handset is used.
FIG. 1 is a block diagram showing a conventional voice switch. The voice switch therein illustrated comprises an attenuator 12 provided in the transmit speaking circuit to which a microphone 10 is connected and an attenuator 16 provided in the receive speaking circuit to which a speaker 18 is connected. Further, the voice switch includes a voice power calculator 14 for calculating a power P.sub.T of a transmitted voice supplied by the microphone 10; a voice power calculator 20 for calculating a power P.sub.R of a received voice, received via a communication line and an attenuation controller 22 for comparing the above-mentioned voice powers P.sub.T and P.sub.R and controlling the attenuators 12 and 16 by making a determination described in the following.
The attenuation controller 22 controls the attenuation in the following manner. When it is found, in comparing the above-mentioned voice powers, that aP.sub.R .gtoreq.P.sub.T, the attenuation of the attenuator 16 in the receive path is adjusted to be small, and the attenuation of the attenuator 12 in the transmit path is adjusted to be large. When aP.sub.R &lt;P.sub.T, the attenuation of the attenuator 12 in the transmit path is adjusted to be small, and the attenuation of the attenuator 16 in the receive path is adjusted to be large. A constant a is determined on the basis of the estimated degree of acoustic coupling.
The problem with the voice switch of FIG. 1 derives from the inability of the switch to differentiate an echo, generated by a received voice which reaches the microphone 10 after reverberating in the room in which the associated communications set is housed, from the transmitted voice. If, due to the echo, it is found, in comparing the powers of the transmitted voice and the received voice, that aP.sub.R &lt;P.sub.T, the voice switch selects the transmit speech path, thus causing a receive blocking phenomenon to occur in which a subsequent received voice is blocked till the condition aP.sub.R .gtoreq.P.sub.T sets in. (A similar phenomenon caused by the echo of the transmitted voice is called a transmit blocking phenomenon. While the receive blocking phenomenon is taken as an example throughout the description that follows, it is to be noted that the two phenomena are of the same nature). Variations in voice level are fairly large so that it is highly likely that the condition aP.sub.R &lt;P.sub.T occurs.
It is when the received voice level drops to nil that this phenomenon is likely to occur. In such a case, while the received voice level drops to nil and the power P.sub.R becomes small, the echo thereof does not drop to a small level immediately. Therefore, it is very likely that the condition aP.sub.R &lt;P.sub.T sets in.
The occurrence of this phenomenon is countered conventionally, by increasing the constant a, and by increasing the time consonant of a filter employed for calculating the received voice power, so as to become equal to the reverberation time, so that the condition aP.sub.R &lt;P.sub.T will not set in.
However, increasing the time constant of the power calculating filter means increasing a time for which the absence of the received voice, i.e. the mute period, is construed as the time during which the received voice is still being received. Increasing the constant a means increasing the transmitted voice power necessary for the voice switch to select the transmit speech path. Thus, the two arrangements both act to delay the switching of the voice switch to the transmit path, thereby prohibiting smooth switching, as is necessary for a normal conversation in which the parties engaged in the conversation take turns speaking, but without intervals between their respective speech intervals. The value to be assigned to the constant a varies according to the degree of acoustic coupling, thus necessitating that the constant have a relatively large level in anticipation of the largest amount of echo (reverberation) that may be picked up, and thus making it extremely difficult to detect the transmitted voice.
There is proposed in the Japanese Laid-Open Patent 2-309851 a voice switch as shown in FIG. 2 as a solution to the above-mentioned problems. In FIG. 2, those parts that are identical to the parts of FIG. 1 are designated by the same reference numerals from figure to figure. The voice switch shown in FIG. 2 comprises voice detectors (detection units) 24 and 26, a line status determination unit 28 and a line status holding unit 30. A hybrid circuit 32 provides four-wire to two-wire interface between the hands-free system and the communication line.
A voice Level detection circuit 24A of the voice detector 24 detects the transmitted voice level, and a noise level detection circuit 24B detects the delivered from the microphone 10. A comparator 28A of the line status determination unit 28 makes a determination that a voice is delivered when the transmitted voice level is greater than the noise level, thereby selecting the transmitted voice level as an output therefrom. Otherwise, the comparator 28A determines that no voice is delivered and outputs zero. Similarly, a voice level detection circuit 26A of the voice detector 26 detects the received voice level, and a noise level detecting circuit 26B detects the level of a noise delivered via the communication line. A comparator 28B of the line status determination unit 28 selects the received voice level as an output therefrom when the received voice level is greater than the noise level. Otherwise, the comparator 28B outputs zero.
A comparator 30A of the line status holding circuit 30 compares the transmitted voice level from the comparator 28A and the received voice level from the comparator 28B, and operates in accordance with the rules set forth below:
(1) Provide a great attenuation to a signal on the receive speech path and a small attenuation to a signal on the transmit speech path in case only the transmitted voice is detected.
(2) Provide a great attenuation to a signal on the transmit speech path and a small attenuation to a signal on the receive speech path in case only the received voice is detected.
(3) Hold the line status as it is, irrespective of which of the voice levels is greater, in case both the transmitted voice and the received voice are detected.
(4) Hold the line status as it is in case no voice is delivered on either the transmit speech path or the receive speech path.
The above-described results are registered in a register 30B, the attenuators 12 and 16 being controlled on the basis thereof.
Thus, even when the received voice output from the speaker 18 reverberates and this reverberation is interpreted as the transmitted voice, the above-mentioned rule (3) enables the voice switch to continue to select the receive speech path as long as the received voice continues to be detected. In this way, the aforementioned problem of the conventional method, namely a failure to provide correct attenuation due to variations in voice levels, is eliminated.
However, a problem arises with the conventional voice switch as shown in FIG. 2 when the received voice level drops to nil. That is, when the received voice level drops to nil, the reverberation (echo) is interpreted as the transmitted voice, and the voice switch is switched to the transmit speech path. Consequently, the voice switch continues to select the transmit speech path until the reverberation decreases to a level low enough for the reverberation not to be interpreted as the transmitted voice. As a result, the voice received in this state is blocked by the attenuator 16 and is not output from the speaker 18. In other words, receive blocking takes place.